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        <td class="header">&nbsp; Interferogram formation (InSAR operator)</td>
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<h3>Computation of interferogram and computation/removal of the flat-earth phase</h3>

<p>
    This operator computes (complex) interferogram, with or without subtraction
    of the flat-earth (reference) phase. The reference phase is subtracted using a
    2d-polynomial that is also estimated in this operator.
</p>

<p>
    If the orbits for interferometric pair are known, the flat-earth phase is estimated using the orbital
    and metadata information and subtracted from the complex interferogram. The flat-earth phase is the phase
    present in the interferometric signal due to the curvature of the reference surface. The geometric reference
    system of the reference surface is defined by the reference system of satellite orbits (for now only WGS84
    supported, which the reference system used by all space-borne SAR systems).
</p>

<p>
    The flat-earth phase is computed in a number of points distributed over
    the total image, after which a 2d-polynomial is estimated (using least squares) fitting
    these 'observations', (e.g. plane can be fitted by setting the degree to 1.)
</p>

<p>
    A polynomial of degree 5 normally is sufficient to model the reference
    phase for a full SAR scene (approx 100x100km). While, a lower degree might be
    selected for smaller images, and higher degree for 'long-swath' scenes.
    Note that the higher order terms of the flat-earth polynomial
    are usually small, because the polynomial describes a smooth, long wave body (ellipsoid).
    To recommended polynomial degree, that should ensure the smooth surface for most image sizes
    and areas of the world is 5th degree.
</p>

<p>
    In order to reduce the noise, as the post-processing step, you can perform multilooking (with Multilook
    Operator). Multilooking has to be performed separately on 'virtual' bands phase or intensity. In future
    releases complex Multilook operator will be released.
    Note that in case of ESA's ERS and Envisat sensors, the factor 5:1 (azimuth:range) or similar ratio
    between the factors is chosen to obtain approximately square pixels (20x20 m^2 for factors 5 and 1).
    Of course the resolution decreases if multilooking is applied.
</p>

<p>
    <b>Note:</b> The reference phase polynomial is not estimated nor subtracted from the computed interferogram, if the
    flag for NOT performing the reference-phase estimation/subtraction is being marked. Note that if the subtraction of
    the flat-earth is being skipped the formed interferogram will still have the fringes caused by the earth curvature,
    and could hamper further interferometric processing and analysis. The intention of computing interferogram without
    the reference-phase being subtracted is only for demonstration and educational purposes.
</p>

<h4>Operator parameters:</h4>&nbsp;&nbsp;
The following parameters are used by this operator:
<ol>
    <li>
        Number of points to compute reference phase for least square estimation. Default value is 501, and
        sufficient for 100x100km SAR scenes. For smaller/or larger scenes this number can be adapted.
    </li>
    <li>
        The degree of 2D flat-earth polynomial. Recommended degree, appropriate for most usage cases is 5th degree.
    </li>
    <li>
        Orbit interpolation method. Defaults to a polynomial of degree (number of state vectors)-1, but smaller
        than degree 5. Optionally, the degree for the orbit interpolation can be declared. The specified degree has
        to be smaller or equal to (number of state vectors)-1. The positions of state vectors (x,y,z) are
        independently interpolated, the velocities are estimated from the position. The annotated
        velocities, if available, will be used in the interpolation and not computed from the positions.
        NOTE: It is not recommended to use a degree smaller
        than the maximum possible, except if it gets too large to avoid oscillations. However, depending on
        the temporal posting of the state vectors and their accuracy different interpolation strategies can
        give better results.
    </li>
    <li>
        Flag for skipping estimation and subtraction of the reference-phase.</li><li>Flag for subtracting topographic phase</li><li>DEM to be used for subtracting topographic phase</li><li>Extension of tile (%) for DEM simulation (optimization parameter)</li><li>Flag for outputting elevation (if topographic phase subtraction is chosen)</li><li>Flag for outputting orthorectified latitudes and longitudes&nbsp;(if topographic phase subtraction is chosen)</li><li>Flag for including coherence estimation</li><li>Choice of using square pixel or not (for&nbsp;coherence estimation)</li><li>Coherence range and azimuth window sizes</li>
</ol>

<h4>Source bands:</h4>

<p>
    The source bands are the set of, usually coregistered, bands of the complex product
</p>

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